Explained: Radiative forcing

Mar 10, 2010 by David L. Chandler

When people talk about global warming or the greenhouse effect, the main underlying scientific concept that describes the process is radiative forcing. And despite all the recent controversy over leaked emails and charges of poorly sourced references in the last Intergovernmental Panel on Climate Change report, the basic concept of radiative forcing is one on which scientists -- whatever their views on global warming or the IPCC -- all seem to agree. Disagreements come into play in determining the actual value of that number.

The concept of radiative forcing is fairly straightforward. Energy is constantly flowing into the atmosphere in the form of sunlight that always shines on half of the Earth’s surface. Some of this sunlight (about 30 percent) is reflected back to space and the rest is absorbed by the planet. And like any warm object sitting in cold surroundings -- and space is a very cold place -- some energy is always radiating back out into space as invisible infrared light. Subtract the energy flowing out from the energy flowing in, and if the number is anything other than zero, there has to be some warming (or cooling, if the number is negative) going on.

It’s as if you have a kettle full of water, which is at room temperature. That means everything is at equilibrium, and nothing will change except as small random variations. But light a fire under that kettle, and suddenly there will be more energy flowing into that water than radiating out, and the water is going to start getting hotter.

In short, radiative forcing is a direct measure of the amount that the Earth’s energy budget is out of balance.

For the Earth’s climate system, it turns out that the level where this imbalance can most meaningfully be measured is the boundary between the troposphere (the lowest level of the atmosphere) and the stratosphere (the very thin upper layer). For all practical purposes, where weather and climate are concerned, this boundary marks the top of the atmosphere.

This summary chart from the IPCC’s Fourth Assessment Report shows the main components of radiative forcing that affect the Earth’s climate. The values represent the amount these factors have changed (as of 2005) since 1750, in the pre-industrial era, along with the amount of uncertainty associated with each number. Note that by far the largest uncertainties are in the effect of aerosols. Chart: IPCC

While the concept is simple, the analysis required to figure out the actual value of this number for the Earth right now is much more complicated and difficult. Many different factors have an effect on this balancing act, and each has its own level of uncertainty and its own difficulties in being precisely measured. And the individual contributions to radiative forcing cannot simply be added together to get the total, because some of the factors overlap — for example, some different greenhouse gases absorb and emit at the same infrared wavelengths of radiation, so their combined warming effect is less than the sum of their individual effects.

In its most recent report in 2007, the IPCC produced the most comprehensive estimate to date of the overall radiative forcing affecting the Earth today. Ronald Prinn, the TEPCO Professor of Atmospheric Science and director of MIT’s Center for Global Change Science, was one of the lead authors of that chapter of the IPCC’s Fourth Assessment Report. Radiative forcing “was very small in the past, when global average temperatures were not rising or falling substantially,” he explains. For convenience, most researchers choose a “baseline” year before the beginning of world industrialization — usually either 1750 or 1850 — as the zero point, and compute radiative forcing in relation to that base. The IPCC uses 1750 as its base year and it is the changes in the various radiative forcing agents since then that are counted.

Thus radiative forcing, measured in watts per square meter of surface, is a direct measure of the impact that recent human activities — including not just greenhouse gases added to the air, but also the impact of deforestation, which changes the reflectivity of the surface — are having on changing the planet’s climate. However, this number also includes any natural effects that may also have changed during that time, such as changes in the sun’s output (which has produced a slight warming effect) and particles spewed into the atmosphere from volcanoes (which generally produce a very short-lived cooling effect, or negative forcing).

Although all of the factors that influence radiative forcing have uncertainties associated with them, one factor overwhelmingly affects the uncertainty: the effects of aerosols (small airborne particles) in the atmosphere. That’s because these effects are highly complex and often contradictory. For example, bright aerosols (like sulfates from coal-burning) are a cooling mechanism, whereas dark aerosols (like black carbon from diesel exhausts) lead to warming. Also, adding sulfate aerosols to clouds leads to smaller but more abundant droplets that increase cloud reflectivity, thus cooling the planet.

So, given all these factors and their range of errors, what’s the answer? The current level of radiative forcing, according to the IPCC AR4, is 1.6 watts per square meter (with a range of uncertainty from 0.6 to 2.4). That may not sound like much, Prinn says, until you consider the total land area of the Earth and multiply it out, which gives a total warming effect of about 800 terawatts — more than 50 times the world’s average rate of energy consumption, which is currently about 15 terawatts.

Part two of this series will examine the concept of climate sensitivity, which determines how much the planet’s temperature will change due to a given radiative forcing.

This is the first of a two-part “Explained” on the scientific concepts underlying the concept of the greenhouse effect and global climate change. Part 2.

Related Stories

A group of scientists affiliated with the International Geosphere-Biosphere Programme (IGBP) have proposed a new framework to account more accurately for the effects of aerosols on precipitation in climate models. Their work ...

Global warming may proceed faster and be more severe than previously predicted according to research about to be published in the scientific journal Nature. Reductions in airborne particle pollution, or aerosols, as air qu ...

(PhysOrg.com) -- Water vapor is known to be Earth's most abundant greenhouse gas, but the extent of its contribution to global warming has been debated. Using recent NASA satellite data, researchers have estimated ...

(PhysOrg.com) -- Aircraft emissions can affect the properties of cirrus clouds, contributing to climate change. This was a key finding from PNNL scientist Dr. Xiaohong Liu and his colleagues from a recent ...

Particles cool down the climate, but to which extent? This has remained an unanswered question for scientists. A new article in Science by Gunnar Myhre at CICERO, Norway, brings the scientific community a step closer to sol ...

Scientists need a more detailed understanding of how human-produced atmospheric particles, called aerosols, affect climate in order to produce better predictions of Earth's future climate, according to a NASA-led report issued ...

Recommended for you

In their open-access paper for Geology, Kimberly Genareau and colleagues propose, for the first time, a mechanism for the generation of glass spherules in geologic deposits through the occurrence of volcan ...

An analysis of buildings tagged red and yellow by structural engineers after the August 2014 earthquake in Napa links pre-1950 buildings and the underlying sedimentary basin to the greatest shaking damage, ...

As everyone who lives in the San Francisco Bay Area knows, the Earth moves under our feet. But what about the stresses that cause earthquakes? How much is known about them? Until now, our understanding of ...

(Phys.org)—A trio of researchers with the Indian Institute of Science has found, via computer simulation, that deforestation in one part of the world can impact rainfall patterns in another. In their paper ...

It's no surprise that Arctic sea ice is thinning. What is new is just how long, how steadily, and how much it has declined. University of Washington researchers compiled modern and historic measurements to ...

@marjoke: Please read the article before you comment to avoid making your normal foolish self apparent.

For convenience, most researchers choose a “baseline” year before the beginning of world industrialization — usually either 1750 or 1850 — as the zero point, and compute radiative forcing in relation to that base. The IPCC uses 1750 as its base year and it is the changes in the various radiative forcing agents since then that are counted.

Does that not make sense to you? I didn't think it was all that difficult.

Where did the data come from? Is it freely and openly available?The solar constant sic has been measured to vary as much the combined effect of the so called greenhouse gasses- and that's only over the past ca. 40 yrs.

"Radiative forcing “was very small in the past, when global average temperatures were not rising or falling substantially,” he explains."With this statement Prinn must still be using Mann et al discredited hockey stick graph (invalidated statistical methods on top of cherry picked tree ring data from a small area of Russia). The "consensus" is that the earth was warmer 800 - 1400 AD and much cooler from ca. 1600 to 1850 AD.

When you catch a person lying to you one time, do you readily believe him the next time he tells you something? If you catch that same person blatantly lying to you many times do you believe him about anything? IMHO - Only if you are a fool or self-delusional.

@marjoke: Please read the article before you comment to avoid making your normal foolish self apparent.

For convenience, most researchers choose a &#65533;baseline&#65533; year before the beginning of world industrialization &#65533; usually either 1750 or 1850 &#65533; as the zero point, and compute radiative forcing in relation to that base. The IPCC uses 1750 as its base year and it is the changes in the various radiative forcing agents since then that are counted.

Does that not make sense to you? I didn't think it was all that difficult.

You actually seem to think that redefining a term used in an article and within climate science somehow makes your comment more valid, even though it was pointed out to you that it was nonsense? When someone comes up to you and tells you that you're comparing apples to oranges, do you then redefine oranges in order for them to be the same thing as apples and make your argument valid?

Once again you've proven that you have no idea what you're talking about and that you're just another turtle on a post.

Note: MBH (Mann, Bradley, Hughes) has never been discredited. The above poster is a troll to ACC/AGW threads and reposts the same claims on a continuous basis without ever citing their sources, or at least a credible source. In response to challenges like that, however, the MBH hockey-stick research was redone with a new series of proxies in 2008 and came to the same conclusion.http://www.meteo....AS08.pdf

You actually seem to think that redefining a term used in an article and within climate science somehow makes your comment more valid, even though it was pointed out to you that it was nonsense? When someone comes up to you and tells you that you're comparing apples to oranges, do you then redefine oranges in order for them to be the same thing as apples and make your argument valid?

Once again you've proven that you have no idea what you're talking about and that you're just another turtle on a post.

What nonsense? Energy is NEVER out of balance.reflectance + transmission + absorption =1.Because of the large uncertainties with the energy radiated from earth, NASA is working on the CLARREO to have a NIST calibrated radiometer measure what is radiated into space.

Where did the data come from? Is it freely and openly available?The solar constant has been measured to vary as much the combined effect of the so called greenhouse gases - and that's only over the past ca. 40 yrs.

In theory, greenhouse gases are supposed to warm the atmosphere by absorbing reflected long wave IR passing through, but over 30 yrs of satellite data shows zero warming.

"Radiative forcing “was very small in the past, when global average temperatures were not rising or falling substantially,” he explains."With this statement Prinn must still be using Mann et al discredited hockey stick graph (invalidated statistical methods on top of cherry picked tree ring data from a small area of Russia). The "consensus", and evidence too, is that the earth was warmer 800 - 1400 AD and much cooler from ca. 1600 to 1850 AD.

IPCC4 is a replete with politically based lies. It's not believable and either is Prinn et al, MIT or not.

Is it really too much to ask for a creditable citation? The statistical methods were not invalid, they just weren't optimum for the analysis done, but using a different analytical method didn't change the results significantly.

"When averaged over a year, the incoming energy in both the earth and its atmosphere equals the outgoing energy.

If we consider the entire Earth-atmosphere system, then the amount of radiation entering the system must equal to the amount leaving, or the system would continually heat or cool. Not all of this energy is radiative energy; some is sensible and latent heat."http://okfirst.me...et2.html

"In short, radiative forcing is a direct measure of the amount that the Earth’s energy budget is out of balance. "This sentence is not correct.

I think physorg posted this article just to get us away from the other articles.

I refuse to debate this article based on the fact that everyone knows radiative forcing is a real element. (If you don't, you certainly shouldn't be speaking about climate in any manner, regardless of your stance).

The contention is in the numbers quantifying said figure, as the article stated, and there isn't a person here who can speak to it with authority, myself included.

@JayK THe hockey stick curve is based on a faulty statistical method. Using Monte Carlo number generation, and plugging those into Mann et al's statistical method one gets a hockey stick 9 times out of 10. The hockey stick is dead - a great example of garbage in/garbage out.You can believe what you want but facts are facts (they're not beliefs).

@deatopmq: And you're still wrong. The hockey stick from MBH98 has been verified over multiple proxies and statistical methods. All you're doing is making a statement with nothing to back it up. If you have something, post the citation, otherwise you're just making noise with nothing but your own flute. I'd at least expect you to mention McIntyre and McKitrick or even Soon and Baliunas as backup to your assertions. Maybe you could even mention the scientific genius of Inhofe.

But as I've stated in many threads, you actually have no proof of your complaint, you just keep repeating it, hoping that people will quit telling you that you're nothing more than a common liar.

There are some things that every single scientist agree on, those that suspect AGW is unproven, and those that believe that AGW is real and dangerous.

It has nothing to do with belief, it has to do with definitions of terms. Radiative forcing is a defined term.

This article was very straightforward in describing the uncertainties involved in the calculation of the current level of radiation forcing. Scientists all around the globe are trying to get a better handle on measuring aerosol concentrations, types and distributions. As time goes on the numbers will be better defined.

Show me all of the data, with error margins, for 1750. The only problem will be that the error margin will be far greater than the "radiative forcing". By the way, what kind of term is that (ie. forcing)?!

Now I'll let the reader follow all the links and such, but the general gist of it is that the RF for 1750 vs. 1996 are based on solar minimals and give a high confidence factor.

Well done, the only piece you're omitting, (probably accidentally) is that it gives a high degree of confidence for particular solar phase states. Now that we know how variable the sun is, the calulations get far more complex.

Before any AGW opponents misread and jump on my commentary, recognize that although the computations are more complex, the only change you'll see is in a matter of minor degree and variance as opposed to flat out full spectrum changes.

Yeah, I tried to find that article, Skeptic, but it didn't jump out at me. I'm betting that it is in the citations for one or more of the pieces I posted, and the confidence levels are discussed in the IPCC 2001 WG1 piece, and how the year 1750 is used for the greatest confidence level.

AS I mentioned on a previous post, the global radiative imbalance since 1960 is the equivalent of over 700,000 Little Boy atomic bombs going off every day (though obviously without the dust and radiation issues!)

AS I mentioned on a previous post, the global radiative imbalance since 1960 is the equivalent of over 700,000 Little Boy atomic bombs going off every day (though obviously without the dust and radiation issues!)

AS I mentioned on a previous post, the global radiative imbalance since 1960 is the equivalent of over 700,000 Little Boy atomic bombs going off every day (though obviously without the dust and radiation issues!)

That's not really that much energy seeing as you're talking about the whole globe. I think you're underestimating.

That's less than 0.2 W/M^2, substantially less than the variation in the total solar energy received by Earth from day to day.

AS I mentioned on a previous post, the global radiative imbalance since 1960 is the equivalent of over 700,000 Little Boy atomic bombs going off every day (though obviously without the dust and radiation issues!)

That's not really that much energy seeing as you're talking about the whole globe. I think you're underestimating.

That's less than 0.2 W/M^2, substantially less than the variation in the total solar energy received by Earth from day to day.

So you think the energy released by 700,000 Little Boys....EVERY DAY is not much..hmm So the equivalent energy from over a quarter of a trillion of these bombs every year is not much over the last 50 years. Sure seems to me that will melt a hell of a lot of ice!

So you think the energy released by 700,000 Little Boys....EVERY DAY is not much..hmm So the equivalent energy from over a quarter of a trillion of these bombs every year is not much over the last 50 years. Sure seems to me that will melt a hell of a lot of ice!

How much is it? How many joules?The sun hits the earth with 1345 W/m2.The cross section of the earth is ~1e14 m^2.How many bombs equal 1*10^17 Watts?

So you think the energy released by 700,000 Little Boys....EVERY DAY is not much..hmm So the equivalent energy from over a quarter of a trillion of these bombs every year is not much over the last 50 years.

Mary, you show your ignorance in the subject when you make alarmist claims as the above.

How large do you think the surface area of the atmosphere is?

Math time.

Surface area of the planet at ground level: 510,072,000 km^2Energy released by a single "little boy": 60 TJ or 60,000,000,000,000 joules.

60Tj x 700,000 = 42000000 TJ of energy.

42000000 TJ / 510,072,000 km^2= 0.082 TJ/KM^2

Now the fun part:1TJ = 1,000,000,000,000 J1J= the amount of heat a person emits every 0.01 seconds.

So to make this relevant to you. The amount of energy you're alerting us to on a daily basis is equal to the amount of heat energy that a family of 5 would naturally give off over the course of 63 years.

You say I show 'ignorance'....how? What I used was a way to show the amount of energy required to heat the Earth )0.5C since 1960 ( a conservative estimate).This, unlike your illustration is extra energy put into the system, don't forget, though tempting for you I'm sure, that your family of five, require energy from the system to produce the the heat energy. Of course you would require 102,014,400 of your families over 65 years to generate the extra heat retained by the Earth every day due to AGW. Over one year you would require the 65 year heat output from 37235256000 families (or 186,176,280,000 people or 26 times the current population)Now tell me how many Olympic size swimming pools would be evaporated in one day by 700,000 Little Boy nuclear devices?

Even at that, equal to one half of a percent of total irradiant power per day, multiplied by 365 days equals nearly 2 more days worth of additional heating per year. Can't exactly call that negligible, and you can't attribute it to any other cause than human activity.Just sayin'.

And this radiative forcing estimate from 2005 and earlier is based on facts - like we measured a large part of the radiation input and output of our planet from space - or on computer models?

The indicated margin of error is as large as the indicated increase in watts per square meter. This isn't a strong indication of a significant result.

And the result according to the IPCC shouldn't be 1.6 watts per square meter, it should be 0.6 watts per square meter to present a 'sure' result.(When your speed is measured by the police the measurement uncertainty is subtracted from the measured speed! This is a normal scientific procedure to be certain about the result, and to ensure society doesn't pay an exorbitant amount of money for something we don't know (for sure).)

When chemists can be surprised by unexpected chemical properties of a 'simple' water molecule (2010), the IPCC seems remarkable sure about the unknowns. But that's science, the appendix can be removed safely, or can it?

And this radiative forcing estimate from 2005 and earlier is based on facts - like we measured a large part of the radiation input and output of our planet from space - or on computer models?

The indicated margin of error is as large as the indicated increase in watts per square meter. This isn't a strong indication of a significant result.

And the result according to the IPCC shouldn't be 1.6 watts per square meter, it should be 0.6 watts per square meter to present a 'sure' result.(When your speed is measured by the police the measurement uncertainty is subtracted from the measured speed! This is a normal scientific procedure to be certain about the result, and to ensure society doesn't pay an exorbitant amount of money for something we don't know (for sure).)

When chemists can be surprised by unexpected chemical properties of a 'simple' water molecule (2010), the IPCC seems remarkable sure about the unknowns. But that's science, the appendix can be removed safely, or can it?

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.

Javascript is currently disabled in your web browser. For full site functionality, it is necessary to enable Javascript.
In order to enable it, please see these instructions.